Project Summary/Abstract Internalizing problems emerging during preschool age are associated with future difficulties in academic achievement, deficits in social and peer relations, and risk for psychopathology throughout the lifespan. Yet, relative to other forms of socioemotional dysfunction such as externalizing problems, the early developmental pathways leading to internalizing problems have received less attention. The overall objective of the present study is to investigate family pathways that unfold during pregnancy and the highly sensitive period of infancy and toddlerhood that ultimately impact internalizing problems during preschool age, and to examine the differential effects of these pathways for children with varying degrees of temperamental fearfulness. Based on prior theory and empirical evidence, it is expected that interparental relationship quality during pregnancy will be associated with preschool internalizing problems. Further, it is hypothesized that the impact of the interparental relationship on internalizing problems during preschool will be mediated by both mother-child and father-child relationship quality during infancy and toddlerhood. Lastly, it is expected that the association between parent-child relationship quality during infancy and toddlerhood and internalizing problems at preschool age will be stronger to the extent that children are higher in observed temperamental fearfulness. To test these hypotheses, the integrative, comprehensive construct of mutually responsive orientation (MRO) will be observed in the interparental relationship during pregnancy and in both mother-child and father-child relationships during infancy and toddlerhood as predictors of child internalizing problems during preschool age. MRO captures a dyadic level of relationship quality comprised of coordinated routines, harmonious communication, mutual cooperation, and emotional ambience (high MRO) and maladaptive qualities of the relationship such as hostile communication, lack of responsiveness, and high levels of negative affect (low MRO). Study aims will be pursued in an established sample of 159 families (mother, father, and child) who have completed observational paradigms during pregnancy and are currently completing observational paradigms when the target child turn 1 and 2 years of age. Lastly, mothers and fathers will be invited to complete a survey of child internalizing problems when children turn 3.5 years of age. Findings are expected to improve intervention and prevention efforts for preschoolers with internalizing problems by identifying modifiable factors within the early family environment that ultimately impact the development of internalizing problems.